From one aspect this invention relates to a straightening machine suitable for straightening bars. Bar straightening machines usually comprise at least two rolls between which a bar is passed, the rolls exerting on the bar a bending moment sufficient to cause plastic flow of the material of which the bar is formed. The bar is rotated as it passes between the rolls so that the stress to which each part of the bar is subjected is reversed at least once and usually a number of times as that part of the bar passes between the rolls. Because tubes can be deformed more easily than bars, owing to partial collapse of a tube, tube straightening machines usually comprise several pairs of rolls; whereas bar straightening machines usually comprise only a single pair of rolls. However, it will be understood that a bar straightening machine may be used for straightening either bar or tube and that a tube straightening machine may similarly be used for straightening either bar or tube.
Both of the bar-engaging rolls of a bar straightening machine are normally driven and arranged with their respective axes inclined to the path along which the bar is moved, so that the rolls cause the bar to advance along the path and also to rotate about its axis.
At any given moment a certain length of a bar which is being straightened in a bar straightening machine is stressed beyond its elastic limit. This length is called the plastic length. Experience has shown that the plastic length affects the accuracy with which a bar is straightened. In general, the greater the plastic length, the straighter the resulting bar.